The 37-year-old is leading the strongest challenge in years to President Alexander Lukashenko, a former Soviet collective farm boss who has brooked little opposition in his 26-year rule of the eastern European country.
Supporters cheered and waved flags as Tikhanouskaya addressed the crowd.
“I am not a politician and I do not need (political) power. But my husband is in jail. I had to hide my children for their safety. I sacrificed a peaceful life for all of our sakes. I am tired of being patient. I am tired of being silent. I am tired of being afraid,” she said.
The opposition candidate on Thursday denied allegations that she or her jailed husband had ties to an alleged plot by Russian mercenaries to destabilise Belarus ahead of a presidential election.
Belarusian authorities say they have launched a criminal case against her husband Syarhei Tsikhanouski on suspicion of inciting riots and state-run media suggested he was part of a plot to sow instability.
Tikhanouskaya launched her candidacy in place of her husband, a popular blogger who was detained in May. She was originally reluctant to run after receiving an anonymous threat to have her children taken away from her if she did.
But in recent days she has gathered crowds of thousands of people, amid swelling anger against Lukashenko over his hands-off handling of the coronavirus pandemic and grievances over the economy and human rights.
Lukashenko has allowed little dissent in over a quarter of a century in power in the country of 9.5 million between Russia and European Union member state Poland.
The 65 year old president is expected to win the election despite growing opposition. He portrays himself as a guarantor of economic and political stability.